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bioweb.sungrant.org » Technical » Bioproducts » Bioproducts from Syngas » Isosynthesis

Products from Syngas—Isosynthesis (Catalysts)	Printer Friendly

In its simplest form, syngas (also called producer gas, town gas, blue water gas, and synthesis gas among others) is composed of carbon monoxide (CO) and hydrogen (H2), which provide the building blocks to produce a number of organic compounds. During the 1800’s, coal gasification was used for lighting and heating. The production of fuels and chemicals from syngas began in the early 20th century. Today, methanol and ammonia are produced from syngas, and in addition to hydrogen, constitute the major uses of syngas.    In principle, syngas can be produced from any hydrocarbon feedstock, including natural gas, naphtha, residual oil, petroleum coke, coal, and biomass. Under today’s conditions, the least expensive feedstock is natural gas. Using numerous synthesis pathways, a large number of organic compounds can be produced from syngas (figure 1).                   The isosynthesis reaction converts syngas to isobutene and isobutane under relatively extreme reaction conditions using a thorium or zirconium-based catalyst. Isosynthesis is selective to i-C4 hydrocarbons and only trace amounts of oxygenates (water, methanol, isobutanol, DME, etc.) are formed under isosynthesis reaction conditions. Selective formation of branched hydrocarbons also occurs in isosynthesis.

Isosynthesis reactions
Isosynthesis conditions are optimized for isobutene/isobutane production. At lower temperatures, alcohols and other oxygentates are formed. At higher temperatures, methane and aromatics are formed.   Iisosynthesis reactions involve 2 chain growth mechanisms - a step-wise CO insertion reaction and a condensation reaction involving surface adsorbed oxygenates. The precursor to the oxygenate products has been identified as a surface adsorbed methoxide species (Maruya, 1996). Oxygenates, alcohols and ethers are more than likely the primary reaction products that undergo dehydration and hydrogenation to form the iso-alkenes and branched alkanes.  These two competing chain growth mechanisms result in a discontinuity in the ASF distribution at C4 that explains the relatively high selectivity of the isosynthesis reaction to C4 products.   Isosynthesis reactions generate large quantities of CO2, likely due to the reaction of H2O and CO in the water-gas shift (Su, 2000a, 2000b).  CO2 recycle and reuse should be investigated to improve conversion efficiency to products.

Isosynthesis catalysts
The first catalysts used were thorium based (ThO2). At 150 atm and 450°C, 46% of CO is converted using unpromoted thoria catalysts, but only 10% is isobutene in the C4 fraction that is mostly isobutane (Sofianos, 1992). At higher pressures, dimethyl ether (DME) was the main product. Thorium-based catalysts are good alcohol dehydration catalysts and are the most active isosynthesis catalysts. They have long lifetimes because they can be regenerated by oxidizing the accumulated coke (carbon) that deposits on the surface. Thoria catalysts are not poisoned by sulfur impurities in the syngas and have high resistance to other poisons as well. However, they are radioactive which precludes their commercial use.   Zirconium-based catalysts also have high activity for isosynthesis.  Unpromoted zirconium catalysts have demonstrated 32% CO conversion at 150 atm and 450°C with much higher selectivity to isobutene (Sofianos, 1992) compared to the thorium-based catalysts.  The overall activity of ZrO2-based catalysts for isosynthesis is lower than ThO2-based catalysts.   Various promoters have been investigated to improve the activity and selectivity of ThO2 and ZrO2 catalysts (Jackson, 1990a, 1990b). The most active isosynthesis catalyst is 20% Al2O3/ThO2.  Other promoters such as Zn, Cr and alkali metals have also been tested. The addition of alkali metals to zirconium catalysts had a negative effect on catalyst performance (Jackson, 1990b; Li, 2001a).   Doped zirconium-based catalysts possess oxygen vacancies in the oxide lattice. The most active catalysts tend to have maximum ionic conductivity suggesting that vacancies in the crystal lattice play an important role in the isosynthesis reaction (Li, 2001b, 2002). These oxygen vacancy sites are required for methoxide formation on the catalyst surface that contributes to the condensation reaction (Jackson, 1990a).   The selectivity of the isosynthesis reaction depends on the nature of the active catalyst sites, including oxygen vacancies on the surface, and the number of acidic and basic sites. The balance between acidic and basic catalyst sites dictates overall activity and selectivity (Li, 2002). Enhancing the number of acidic sites on the ZrO2 catalyst increases activity and selectivity to linear C4 hydrocarbons. Increasing the number of basic sites on the catalyst increases the yield of iso-C4 hydrocarbons (Li, 2001b). The acidic catalyst sites are thought to promote condensation and dehydration reactions. The basic sites are known to catalyze CO insertion reactions. The activity of promoted isosynthesis catalyst systems is related to the acid/base ratio, which can be altered and controlled by varying the preparation procedure for mixed oxide catalysts (Feng, 1994, 1995).

Commercial production of isosynthesis products
The isosynthesis process is not currently commercially practiced. Initial research in the early 1940s was with the intent to produce high octane gasoline, but with the development of the petroleum industry, interest in isosyntheis processes waned (Sofianos, 1992). Renewed interest began in the 1990’s resulting from a shortage of petroleum-derived isobutene which was used to produce MTBE, a gasoline additive (Sofianos, 1992). Current research focuses on development of ZrO2-based catalysts for increased activity and selectivity to i-C4 products.   Laboratory studies have been conducted in gas-solid fixed bed reactors, and slurry reactors are also being investigated. Selectivities to C4 products are reported to be higher in slurry reactors compared to fixed bed reactors (Erkey, 1995).

References
Erkey, C.; Wang J. H.; Postula, W.; Feng, Z. T.; Philip, C. V.; Akgerman, A. and Anthony, R. G. (1995). "Isobutylene Production from Synthesis Gas over Zirconia in a Slurry Reactor." Industrial & Engineering Chemistry Research, 34(4), 1021-1026. Feng, Z. T.; Postula, W. S.; Akgerman, A. and Anthony, R. G. (1995). "Characterization of Zirconia-Based Catalysts Prepared by Precipitation, Calcination, and Modified Sol-Gel Methods." Industrial & Engineering Chemistry Research, 34(1), 78-82. Feng, Z. T.; Postula, W. S.; Erkey, C.; Philip, C. V.; Akgerman, A., and Anthong, R. G. (1994). "Selective Formation of Isobutane and Isobutene from Synthesis Gas over Zirconia Catalysts Prepared by a Modified Sol-Gel Method." Journal of Catalysis, 148(1), 84-90. Jackson, N. B. and Ekerdt, J. G. (1990a). "Isotope Studies of the Effect of Acid Sites on the Reactions of C-3 Intermediates During Isosynthesis over Zirconium Dioxide and Modified Zirconium Dioxide." Journal of Catalysis, 126(1), 46-56. Jackson, N. B., and Ekerdt, J. G. (1990b). "The Surface Characteristics Required for Isosynthesis over Zirconium Dioxide and Modified Zirconium Dioxide." Journal of Catalysis, 126(1), 31-45. Li, Y. W.; He, D. H.; Cheng, Z. X.; Su, C. L.; Li, J. R. and Zhu, Q. M. (2001a). "Effect of calcium salts on isosynthesis over ZrO2 catalysts." Journal of Molecular Catalysis a-Chemical, 175(1-2), 267-275. Li, Y. W.; He, D. H.; Yuan, Y. B.; Cheng, Z. X., and Zhu, Q. M. (2001b). "Selective formation of isobutene from CO hydrogenation over zirconium dioxide based catalysts." Energy & Fuels, 15(6), 1434-1440. Li, Y. W.; He, D. H.; Yuan, Y. B.; Cheng, Z. X., and Zhu, Q. M. (2002). "Influence of acidic and basic properties of ZrO2 based catalysts on isosynthesis." Fuel, 81(11-12), 1611-1617. Maruya, K.; Takasawa, A.; Haraoka, T.; Domen, K., and Onishi, T. (1996). "Role of methoxide species in isobutene formation from CO and H- 2 over oxide catalysts - Methoxide species in isobutene formation." Journal of Molecular Catalysis a-Chemical, 112(1), 143-151. Sofianos, A. (1992). "Production of Branched-Chain Hydrocarbons Via Isosynthesis." Catalysis Today, 15(1), 149-175. Spath, P.L. and D.C. Dayton, Preliminary screening—technical and economic assessment of synthesis gas to fuels and chemicals with emphasis on the potential for biomas-derived syngas, National Renewable Energy Laboratory, NREL/TP-510-34929, December, 2003. Su, C. L., He, D. H.; Li, J. R.; Chen, Z. X., and Zhu, Q. M. (2000a). "Influences of preparation parameters on the structural and catalytic performance of zirconia in isosynthesis." Journal of Molecular Catalysis a-Chemical, 153(1-2), 139-146. Su, C. L.; Li, J. R.; He, D. H.; Cheng, Z. X., and Zhu, Q. M. (2000b). "Synthesis of isobutene from synthesis gas over nanosize zirconia catalysts." Applied Catalysis a-General, 202(1), 81-89.